Japanese honeysuckle presents a serious problem to the economically attractive natural regeneration of loblolly and shortleaf pine. This research investigated the potential allelopathic interference mechanisms of Japanese honeysuckle in relation to pine regeneration and growth, which may provide insight into overcoming this problem. The allelopathic potential of root exudates and leaf litter from Japanese honeysuckle was tested against loblolly and shortleaf pine seedlings. When Japanese honeysuckle and loblolly pine seedlings were grown using the same irrigation reservoir, there was no significant effect on the growth of either pine species. Exudates of Japanese honeysuckle grown as a pure culture in donor cups also produced no growth effects on pure-cultured pine seedlings grown in acceptor cups. In other assays, aqueous extracts of Japanese honeysuckle leaf tissue were toxic to duckweeds at concentrations well below levels where plasmolysis might cause effects. When Japanese honeysuckle leaf tissue was added to soil at a rate of 2 g tissue 100 g⁻¹ soil, mean seedling height at 128 d after planting was reduced by as much as 40%. Moreover, pine seedlings grown in the presence of Japanese honeysuckle tissue exhibited significant chlorosis of the shoot and needles. Gas chromatography–mass spectroscopy analyses and high-performance liquid chromatography of Japanese honeysuckle leaf tissue aqueous extracts confirmed the presence of five compounds previously identified as possible allelochemicals: 4-hydroxycinnamic acid; 2-hydroxycinnamic acid; 3,4-dihydroxybenzoic acid; 3,4-dihydroxycinnamic acid; and chlorogenic acid. Results indicate that allelopathy plays at least a partial role in Japanese honeysuckle interference with loblolly and shortleaf pine.

Nomenclature: Japanese honeysuckle, Lonicera japonica Thunb.; loblolly pine, Pinus taeda L.; shortleaf pine, Pinus echinata Mill; duckweed, Lemna minor L.